This invention relates to an improved electrochromic device which utilizes a combination of a transition metal oxide that takes on color in its electrochemically reduced state and another electrochromic material that takes on color in its electrochemically oxidized state, and to a method of producing the same.
It is known to use a combination of the aforementioned two types of electrochromic (EC) materials in an EC device for display or other purposes. More particularly, such an EC device has oppositely arranged two electrode layers each of which is comprised of a electrode film deposited on a substrate and a coating layer of an EC material, and the two types of EC materials are assigned to the two electrode layers, respectively.
For example, as shown in JP-A No. 62-37247 (Feb. 18, 1987) which relates to an EC device for use as a variable reflectance nonglaring mirror, a transition metal oxide which is colorless in its electrochemically oxidized state and takes on a deep color by electrochemical reduction, such as WO.sub.3, and a polymer of a conjugated compound, such as polytriphenylamine, which is colorless in its electrochemically reduced state and takes on color by electrochemical oxidation are used in combination. The space between the two EC electrode layers is filled with an electrolyte liquid such as a solution of lithium perchlorate in propylene carbonate. In operation of the EC device, electrochemical oxidation of the EC material of one electrode is accompanied by electrochemical reduction of the EC material of the opposite electrode. Accordingly simultaneous coloration of the two electrodes and simultaneous bleaching of the two electrodes take place. The primary purpose of this construction is intensifying coloration of the EC device.
In this type of EC devices it is usual to add a small quantity of water to the electrolyte solution using an organic solvent. As described, for example, in U.S. Pat. No. 4,348,077 the purpose of adding water is to enhance the speed of response of the transition metal oxide film to voltages applied thereto. However, the existence of a substantial amount of water in the electrolyte solution raises a problem that the transition metal oxide film gradually dissolves in the solution by the action of water and, therefore, undergoes a decrease in the qnantity of charge it acquires or delivers in electrochemical oxidation and reduction reactions. For this reason the EC devices are not yet satisfactory in durability.